The present invention relates generally to a method of recovering sulfuric acid from a mixture of sugars and concentrated sulfuric acid, and more particularly, to a method of recovering sulfuric acid from the hydrolyzate product obtained from the acid hydrolysis of biomass.
As a result of the continual depletion of nonrenewable energy sources and the rapid escalation of energy prices, various energy conservation measures have been encouraged and alternative energy sources have been proposed and studied. Among the proposed alternative enegy sources, current research has focused heavily on the use of biomass as a viable energy source.
Biomass is composed of three major materials: cellulose, hemicellulose and lignin in ratios of roughly 4:3:3. This composition enables biomass to be a versatile alternative energy source since cellulose is a polymer of hexose rings and hemicellulose is a polymer composed of hexose and pentose rings. These polymer chains may be broken and converted to sugars and other chemicals through various chemical, microbial or fermentation processes.
The principal sources of lignocellulosic biomass are agricultural crops, agricultural residues, forest products, and municipal waste. Biomass is particularly attractive as an alternative energy source since it is available in large quantities and is renewable. It can also be converted to a variety of chemicals and its conversion need not create air pollution problems. Instead, such conversion can assist in alleviating municipal waste problems.
To optimize the conversion of biomass to energy producing fuels and other valuable chemicals, it is necessary to fractionate the crude biomass to the sugar monomers: glucose and xylose. The most common method used in accomplishing this conversion is acid hydrolysis. In general, the acid hydrolysis of biomass requires either high temperatures and dilute acid or high acid concentrations and low temperatures to obtain acceptably high sugar yields. The high temperature/dilute acid process has the advantage of not requiring acid recovery, because of the relatively inexpensive dilute acid being utilized. However, this process has the disadvantage of suffering low sugar yields caused by the degradation of sugars at the high temperatures needed.
The high acid concentration/low temperature process produces high sugar yields, but the economic success of the process requires acid recovery. Accordingly, it is recognized that for the conversion of biomass to sugars and other usable chemicals through the use of concentrated acids to be economically feasible, the process must include an efficient acid recovery procedure. Typically, dilute acid processes involve acid concentrations of 5% or less, while concentrated acid processes involve acid concentrations of 10% or more.
The principal method of treating the hydrolyzate solution containing sulfuric acid and sugars has been neutralization with lime and removing the sulfate salts as gypsum. A more economical method of recovery is by diffusion dialysis or electrodialysis. The Hokkaido process, commercialized in Japan in the early 1960's, reportedly utilized this technology. The disadvantages of this method include the high capital costs for the equipment and the difficulties associated with both obtaining complete acid/sugar separation and a high acid concentration.
Accordingly, there is a need in the art for a method of recovering concentrated sulfuric acid from a sugar/sulfuric acid product obtained from the acid hydrolysis of biomass or other cellulose materials. A further need is a method to separate concentrated sulfuric acid and still permit subsequent recovery of the sugars.